Automatic precision pipetting device

ABSTRACT

The invention relates to an automatic precision pipetting device. The inventive device comprises at least two cylinder/piston-type pumping units ( 1, 2 ), the rods of which are rotated by means of a common motor unit (MP). Moreover, the pumping chamber of each of the pumping units ( 1, 2 ) is connected to a circuit comprising, successively, a conduit that opens into a container of rinsing liquid (RL), three successive solenoid valves (EV 1 , EV 2 , EV 3 ) and a portion of circuit which connects the second solenoid valve (EV 2 ) with the pipetting means (AP). The larger pumping chamber is connected in the part of the circuit that provides the connection between the two solenoid valves (EV 1 , EV 2 ) while the second pumping chamber is connected to the above-mentioned portion of circuit by means of a connector, said portion of circuit comprising a solenoid valve (EV 3 ) which is disposed between said connector and the aforementioned pipetting means. The invention is suitable for reagent reconstitution, in particular, in an automatic analysis device.

The present invention concerns an automatic precision drawing-off devicewith rinsing of the pipette, this device making it possible to restorereactive agents and being able to be used in an analysis robot.

It more particularly concerns a device of this type having a modularstructure enabling it to be easily adapted according to the requiredprecision and specifications, both as regards the quantities of productssampled in the pipette as well as the quantities of rising liquid used.

Generally speaking, there already exist a large number of devices ableto carry out drawing-off and rinsing cycles, especially inside anautomatic analysis device.

Usually, these devices introduce at least two motorisations, one beingused to activate a doping syringe, the other being used to drive inrotation a pump used for injecting rinsing liquid. In fact, the dopingsyringe, which is provided for small liquid quantities, has aninsufficient capacity to carry out rinsing.

This solution is thus relatively complex and expensive. It introduces apump whose motorisation is expensive in terms of energy and whosefragility and period of life are not as good as those of the syringe.The reliability of the unit is therefore not one would hope to expect.Now, this type of device needs to be able to function without the needfor maintenance for at least seven years at the cycle of the robot onwhich it is used. In the case of a robot such as the one described inthe patent FR 2 779 827, this rate is 60 tests per hour for at least twohours per day 220 days a year (namely about 185,000 tests).

Now it has been observed that the mechanism for activating the syringeis the centre of wear generating in the long run an increasingly largerplay. This is particularly the case when this mechanism comprises aback-geared motor coupled to the rod of the syringe by means of a devicefor converting the movement of rotation of the motor into a linearmovement of said rod. The play is then due to the wear of the teeth ofthe pinions and/or of the racks used both in the back-geared motor andin the conversion device.

Of course, the accuracy of the drawing-off device is affected by thisplay which acts in the way of a hysteresis so as to limit the travel ofthe rod of the syringe at its two extremities. This play appears moreparticularly on each inversion of the direction of rotation of themotor. It is that much more harmful to the accuracy of the device whendigital data relating to the quantities of liquid picked up by thepipette are provided by a digital encoder equipping the motor and as aresult, the mechanical play induces a difference between the volumedetermined according to the data provided by the encoder and the volumeof liquid effectively picked up or put back by the pipette.

Moreover, so as to embody a drawing-off device whose reliability andperiod of life are the same as those of the drawing-off syringe, theApplicant has embodied a drawing-off device comprising at least twopumping units, each comprising a cylindrical cavity inside which arod/piston assembly slides imperviously, said assembly delimiting withthe cavity a working chamber whose volume varies according to the axialposition of the rod/piston assembly.

The extremities of the two rods/pistons coming out of two cavities arecoupled to an activation member driven in translation by a commonmotorisation.

The working chamber of each of the pumping units is moreover connectedto a circuit successively comprising a pipe opening into a liquidrinsing reserve, two successive electrovalves and a tube, possiblyflexible, connected to drawing-off means, such as a needle.

The largest working chamber is then connected into the circuit portionensuring the joining point between the two electrovalves, whereas theother working chamber is connected into the circuit portion situatedbetween the second electrovalve and the drawing-off means.

This device further comprises means for controlling the motorisation andelectrovalves designed so as to provide a cycle comprising at least:

-   -   a drawing-off phase in which the first electrovalve is open, the        second electrovalve is closed and the motorisation drives in        translation the two rod/piston assembles so as to increase the        volume of the two working chambers, the volume increase of the        small chamber generating the sucking up of the liquid to be        analysed or sucking up of the reactive agent in the drawing-off        means, whereas the volume increase of the large chamber provokes        sucking up of the rinsing liquid inside this chamber,    -   a flow back phase in which the two electrovalves are in the same        state as during the drawing-off phase, the motorisation then        acting so as to provoke a reduction of the volumes of said        working chambers and a flowing back of the reactive agent or the        liquid to be analysed,    -   a rinsing phase in which the first electrovalve is closed        whereas the second is open, the motorisation driving in        translation the two rod/piston assemblies so as to reduce the        volume of the two working chambers by expelling the rinsing        liquid they contain towards the drawing-off means.

So as to eliminate the drawbacks due to mechanical plays, the inventionprovides an additional electrovalve placed in the circuit connecting thesecond pumping unit to the pipette and only opens this electrovalve soas to only carry out the sampling phase and/or flow back phase when therod/piston assemblies are currently moving in either direction after theinversion or starting transitory phases and/or stoppage the transitoryphases.

By means of this disposition, the teeth of the pinions and the racks ofthe kinematic chain, which mutually gear, are firmly in support againstone another and thus the mechanical play (even if it exists) does notappear during these periods.

Of course, the device of the invention could comprise a number n ofpumping units whose rod/piston assemblies are connected to a givenactivation member and whose working chambers are respectively connectedto a circuit comprising a number n of electrovalves in seriesrespectively connected in the circuit portion ensuring the joiningpoints between the electrovalves concerning the n−1 first valves, thesmall working chamber from the nth valve being connected to thedrawing-off means by means of a circuit comprising an n+1^(st)electrovalve. Said control means are then designed so that in each ofsaid phases a specific number i of electrovalves are found in a closedstate, whereas the other valves, namely a number n-i, are found in theopen state.

Advantageously, the device of the invention could comprise a pluralityof modules each comprising one pumping unit of the above-mentioned typewhose working chamber is connected to a circuit portion comprising oneelectrovalve. This circuit portion then comprises at each of itsextremities means for connecting to the circuit portion of anothermodule and/or to the pipe opening into the rinsing liquid reserve and/orto the tube connected to the drawing-off means. The coupling meansbetween the motorisation and the rod/piston assemblies are then designedto allow coupling of the number of the desired modules.

Embodiments of the invention are described hereafter and given by way ofnon-restrictive examples with reference to the accompanying drawings inwhich:

FIG. 1 is a skeleton diagram of a drawing-off device according to theinvention and using two syringes;

FIG. 2 is a timing diagram of a complete operating sequence of thedrawing-off device shown on FIG. 1;

FIG. 3 is the timing diagram of a simplified operating sequence variantof the drawing-off device shown on FIG. 1;

FIG. 4 is a diagrammatic cutaway view of an embodiment of the deviceshown on FIG. 1;

FIG. 5 is an exploded perspective view of the embodiment shown on FIG.4;

FIG. 6 is a perspective view of the device of FIG. 5 in an assembledstate;

FIG. 7 is a diagrammatic section of a modular pumping unit able to beused in a drawing-off device according to the invention;

FIG. 8 is a diagrammatic representation of the pumping unit of FIG. 6.

In the example shown on FIG. 1, the drawing-off device comprises twopumping units 1, 2 each comprising a cylindrical body C, C′ in which apiston P, P′ move, said body delimiting with a bottom F, F′ a workingchamber with a variable volume.

This piston is integral with a rod T, T′ coming out of the body on theside opposite the bottom F, said rod being coupled to a translationactivation mechanism introducing:

-   -   a coupling element AC to which the rods T and T′ are secured        (there is play between T, T′ and AC so as to mitigate the        parallelism defects),    -   a rack CR integral with the coupling element AC which extends        parallel to the axis of the cylindrical bodies C, C′,    -   a pinion PN driven by a step motor MP which gears with the rack        CR.

The bottom of each of the bodies C, C′ is provided with a pipe CO, CO′making the corresponding working chamber communicate with a circuitcomprising in series a pipe CP1 opening into a liquid rinsing reserveRL, three successive electrovalves EV₁, EV₂ and EV₃ and a flexible tubeTS connecting the electrovalve EV₃ to a mobile drawing-off needle AP.

This needle AP is activated so as to be able to be engaged in variousreceptacles, such as a reserve RE containing a sample or reactive agent,an analysis receptacle RA and a rinsing well PR.

More specifically, the pipe CO is connected to the circuit connectingthe electrovalves EV₁, EV₂. The pipe CO′ opens into the circuit portionensuring the link between the electrovalve EV₂ and the electrovalve EV₃.

Control of the electrovalves EV₁, EV₂, EV₃ and the motor MP is providedby a microcontroller MC, an optical sensor providing the “zero” positionof the system.

The functioning of the drawing-off device previously mentioned isdescribed hereafter with reference to the timing diagram of FIG. 2.

According to this timing diagram, in an initial state, the needle AP isengaged in the reserve RE, the valves EV₁, EV₂ are located in an openposition whereas the valve EV₃ is in a closed position. The motor MP isat a dead stop, the pistons being in the idle position (position 0). Thetwo working chambers of the pumping units 1, 2 are filled with rinsingliquid.

During a transitory phase preceding drawing-off, the motor MP is drivenin rotation in a negative direction so as to move the two pistons P, P′downwards. This movement creates a sucking up of the rinsing liquid intothe two working chambers. The incidence of the play present in thekinematic chain (which provokes a slight shift in suction) has no effecton the functioning of the device.

The drawing-off phase is then obtained by closing the valve EV₂ and byopening the valve EV₃. In this case, the piston P′ creates suction ofthe liquid contained in the reserve RE, inside the needle AP and aportion of the flexible tube TS, whereas the piston P sucks up therinsing liquid contained in the reserve RL.

At the end of drawing-off, the device moves through a second transitoryphase marked by the closing of the valve EV₃ and the opening of thevalve EV₂ so that drawing off is ended whereas the motor, whichcontinues to rotate, provokes suction by the two chambers of the rinsingliquid derived from the receptacle RL.

At the end of this second transitory phase, the motor MP is stopped andthe needle AP is moved for example, so as to be located above theanalysis receptacle RA.

Once this position is reached, the device starts a third transitoryphase in which the motor MP rotates in an opposite direction (positivedirection) so as to drive the pistons P, P′ towards their idle positions(position 0). During this transitory phase, the valve EV₃ remains closedis whereas the valves EV, and EV₂ are open so as to allow a flowing backof the rinsing liquid towards the receptacle RL.

The flowing back phase is then initiated by opening the valve EV₃ and byclosing the valve EV₂, the valve EV, staying open. During this phase,the piston P′ pushes back the liquid previously sampled in the needle APinside the receptacle RA, whereas the piston P pushes back the rinsingliquid inside the receptacle RL.

The flowing back phase ends by the closing of the valve EV₃ and theopening of the valve EV₂, the valve EV, remaining open.

The motor continues to rotate during a fourth transitory phase and thenis stopped.

The device then starts a phase during which the needle AP is brought tothe right of the rinsing well to allow execution of a rinsing phase.

At the start of this new phase, the valve EV, is closed, whereas thevalves EV₂ and EV₃ are open. The motor MP is activated so as to pushback the rinsing liquid contained in the two syringes in the directionof the drawing-off needle.

In fact, this flowing back is carried out in several stages eachcorresponding to one or several steps of the motor MP.

Once the rinsing phase is carried out, the device starts a return tozero phase with filling of the chambers with the rinsing liquid. To thiseffect, the valves EV₁ and EV₂ are open whereas the valve EV₃ is closed.The motor rotates in an inverse direction (negative direction) so as tobring the pistons back slightly beyond the idle position (position 0) bysucking up the rinsing liquid derived from the receptacle RL.

The device then proceeds to a phase for evacuating air from the needleAP by opening the valves EV₂ and EV₃ and by closing the valve EV₁ and bymaking the motor MP rotate in a positive direction so as to provoke aflowing back of the rinsing liquid towards the drawing-off needle AP andbring the piston and TP₂ into an idle position.

Once this air evacuation phase has been completed, the device goes backto its idle position. The electrovalve EV₃ is then closed, whereas theelectrovalves EV₁ and EV₂ are opened.

The device is then ready to conduct a new operating cycle.

In the simplified sequence shown on FIG. 3, in an initial state thevalves EV₁ and EV₃ are open whereas the valve EV₂ is closed. In fact,this concerns the idle positions (non-excited state) of these valves.The motor is at a dead stop and its angular position is situatedslightly below its zero position.

From this initial state, the motor is driven in rotation in a negativedirection so that the piston P′ generates suction inside the pipette(drawing off of an air bubble), whereas the piston P generates a suctionof the rinsing liquid present in the reserve RL.

The pipette is then engaged in the reactive agent reserve whereas therotation of the motor in a negative direction is accelerated. Adrawing-off phase is then obtained which shall continue for apredetermined period during which the reactive agent is sucked up intothe pipette via the action of the piston P′. This drawing-off phase endsby stoppage of the motor, the closing of the valve EV₃ and then of thevalve EV₂ 30 ms later. The motor starts a transitory phase for invertingthe direction of rotation lasting for a relatively short period.

The rinsing liquid, pushed back by the pistons, then returns to thereserve RL.

Once adjustment of play has been completed, the valve EV₂ is closed andthe valve EV₃ is opened 30 ms later.

The pipette is then moved so as to be brought to the right of theanalysis receptacle RA. Once this position is reached, the device startsa phase for pushing back the product into the analysis receptacle RA,rotation of the motor being accelerated in a positive direction. Thisflowing back phase ends by stopping the motor.

The pipette is then brought to the right of the rinsing well, whereas acontrol is carried out of the zero position of the motor.

The motor is then driven in a positive direction so as to push back theliquid contained in the pipette into the well.

The device then starts a rinsing phase in which the valves EV₂ and EV₃are open whereas the valve EV₁ is closed.

During this phase, the motor caries out a succession of movements ofrotation in a positive direction so as to obtain a flowing back inseveral stages each corresponding to one or several steps of the motor.

At the end of rinsing, the valve EV₃ is closed whereas the valves EV₁and EV₂ are opened. The motor is driven in rotation in a negativedirection so as to provoke a suction of the rinsing liquid by thepistons P and P′. This phase is continued until the position of themotor is slightly below the zero level.

This device then starts a zero control phase during which the motor isdriven in rotation in a positive direction until the zero position isdetected.

The valve EV₁ is closed again and the motor is driven in rotation in anegative direction until the motor returns to a position situatedslightly below the zero level (play adjustment).

The cycle is then ended and the device returns to its initial state, thevalves EV₁ and EV₃ being open, the valve EV₂ closed and the motor MPbeing at a dead stop.

Advantageously, the previously described device could be dimensioned soas to be able to be compatible with currently used analysis robots.

By way of example, in this device used on a robot, such as the onedescribed in the document FR 2 779 827:

-   -   the minimum volume to be drawn off could be equal to 5 μl, the        maximum volume equal to 250 μl, (this volume being determined by        adjusting the number of steps of the motor during the suction        and flowing back phases)    -   for the reactive agent restoration function, the maximum volume        to be drawn off could be equal to 8 ml,    -   the starting flowrate could be 24.4 μl/s or 73.2 μl/s, the upper        flowrate being about 366 μl/s.    -   the device could be able to carry out 10 successive rinsings        with a volume of 150 μl with a period of 100 ms per rinsing, the        pressure of the rinsing stages could be 3 bars,    -   the motor MP used may consist of a step backgeared motor        comprising 200 steps per rotation,    -   the diameter of the piston of the body of the drawing-off unit 1        could be equal to 14 mm, whereas the diameter of the piston of        the body of the drawing-off unit 2 could be 3 mm,    -   the length of the two bores could be 55 mm.

In the example shown on FIGS. 4, 5 and 6, the bodies of the twodrawing-off units 1, 2 are integrated in a given plastic block BL, madefor example of Plexiglas (registered trademark) having an approximatelyparallelepiped shape.

This body comprises two bores AL₁ and AL₂ centered parallel to thevertical axis of symmetry of the block, said bores opening outwardly atthe level of the lower face of the block. In their upper portions, thesetwo bores end by two respective conical portions PC₁, PC₂ situated at apredetermined distance from the upper face.

In the volume between the two bores AL₁, AL₂, a cavity CA is providedopening onto the lower face and onto the front face, as well as avertical passage PV extending between the upper face of the cavity CAand the upper face of the block.

Secured to the lower face of the block is a base EM comprising twotraversing vertical passages in which two respective rod/pistons TP₁,TP₂, made for example of stainless steel, are mounted sliding withimperviousness and respectively engaged in the bores AL₁, AL₂, thesliding imperviousness here being obtained with the aid of dynamicgaskets.

The upper extremities of these rods/pistons are conical, whereas theirlower extremities comprise two respective throats enabling them to befixed at the extremities of the horizontal branch of an invertedT-shaped activation element PA and also be dismantled.

The vertical branch of this activation element PA is fixed at the lowerextremity of a vertical rail RV moving in vertical translation andpassing into the cavity by means of an orifice provided in the base andthen through the passage PV.

This rail RV bears a rack CR on which a pinion PN gears and activated bya back-geared motor (block at broken points MP) and which is located inthe cavity.

Furthermore, three electrovalves EV₁, EV₂, EV₃ are mounted on the frontface of the body in communication with pipes embodied in the bock B inaccordance with the circuit shown on FIG. 1.

An optical fork FO is further provided so as to detect the “zero”position of the rail RV.

The functioning of this device is identical to that previously describedand thus shall not be described again.

Nevertheless, it proves that this solution is particularly advantageousowing to its compactness, ease of integration, its aptitude foreliminating the bubbles by virtue of the conical shapes, its precisionwhich depends on that of the rod/piston TP₁, TP₂ which can be machinedwith extremely high precision, and finally its reliability;

-   -   In particular, the elimination of the bubbles is due to both the        conical shapes of the rod/piston TP₁, TP₂ and of the cylindrical        bores AL₁, AL₂, as well as the surface state of these elements.        Furthermore, the passage of the bubbles is facilitated due to        the fact that the conical shape PC₁ of the cylindrical bore AL₂        with the smallest diameter communicates directly with is the        pipe connected to the drawing-off means AP.

Of course, the invention is not limited to this solution.

It also concerns a modular device implementing pumping modules able tobe assembled in the way indicated on FIGS. 7 and 8.

In this example, each module M₁ to M₄ comprises a cylindrical cavityCC₁, CC₂ in which a rod/piston TP₁, TP₂ is able to slide withimperviousness and activated by a motorisation (block MO) common to allthe rod/piston TP′₁, TP′₂ assemblies.

This module comprises a body having two parallel assembling faces FA₁,FA₂ into which a traversing pipe CT opens in communication with thecylindrical cavity CC, and comprising a portion able to be sealed off bya punch activated by an electromagnet (the unit constituting anelectrovalve EV′₁).

At the level of the assembling faces, the orifices of this pipe CT areequipped with connection means making it possible to provide a sealedconnection of the pipe sections CT of several modules when the latterare assembled together via their assembling faces and fixed in thisposition, for example by tie rods TR.

In a way similar to the foregoing, the pipe obtained by the connectingof the various traversing pipes CT is connected on one side to therinsing liquid receptacle RL and on the other side to a drawing-offneedle AP by means of an electrovalve EV′₃.

The electrovalves EV′₁, EV′₂, EV′₃ and the motorisation MO are connectedto a microprocessor control circuit MC.

Furthermore, each of the modules M₁ to M₄ further comprise a pipe CP incommunication with the cylindrical cavity CC₁ and which opens onto theupper face of the module via an orifice constituting a parallel outletSP. This pipe CP can be sealed off by a punch controlled by anelectromagnet, the unit forming an electrovalve EV′₂ similar to theelectrovalves EV′₁ and controlled by the control circuit.

These parallel outlets SP can be connected to the drawing-off needle APby means of a common collector.

It is clear that this modular structure is extremely flexible and can beadapted to a large number of situations by making the number of modulesvary by selecting modules having cavities with an appropriate diameter,by combining modules whose electrovalves have the same conditions, byselecting the outlets most suitable for the functions it is desired tocarry out, etc. Of course, this selection can be provided by a programmeimplemented by the control circuit MC.

1. Automatic precision drawing-off device with rinsing of the pipette,this device comprising at least two pumping units of different capacity,each comprising a cylindrical cavity inside which a rod/piston unit(slides with imperviousness, said unit delimiting with said cavity aworking chamber whose volume varies according to the axial position ofthe rod/piston unit, the extremities of the two rod/piston units whichcome out of the two cavities being coupled to an activation memberdriven in rotation by a common motorisation, the working chamber of eachof the pumping units being connected to a circuit successivelycomprising a pipe opening into a rinsing liquid reserve, two successiveelectrovalves and a circuit portion connecting the second electrovalveto drawing-off means, the largest working chamber being connected intothe circuit portion providing the link between the two electrovalveswhereas the second working chamber is connected to said circuit portionby means of a connector, characterised in that this circuit portioncomprises an electrovalve situated between said connector and saiddrawing-off means, said electrovalves and said motorisation beingcontrolled by control means designed so as to obtain a drawing-off cyclecomprising at least one suction sequence in which the motor rotatescontinuously in a negative direction so as to provoke a sucking up ofthe rinsing liquid in the two chambers, this sequence comprising atleast the following successive phases: a transitory phase in which thefirst valve is in an open position, a phase for taking a sample in whichthe second valve is closed and the third valve is open, an end of sampletaking phase comprising the closing of the third valve.
 2. Deviceaccording to claim 1, wherein, during the said drawing-off cycle at thetime of said transitory phase, the first two valves are open whereas thethird valve is in a closed position and at the time of said end ofsample taking phase, the third valve is closed and the second valve isopen.
 3. Device according to claim 2, wherein the said drawing-off cyclecomprises a flowing back sequence in which the motor rotates in apositive direction so as to provoke a flowing back of the rinsing liquidin the two chambers, this sequence comprising the following successivephases: a transitory phase in which the third valve is closed whereasthe first two valves are open so as to allow a flowing back of therinsing liquid contained in the chambers towards the receptacle, aflowing back phase in which the third valve is open whereas the secondvalve is closed, the first valve remaining open so as to enable theproduct to flow back into the analysis receptacle, an end of flow backphase comprising the closing of the third valve and the opening of thesecond valve, the valve remaining open.
 4. Device according to claim 3,wherein said drawing-off cycle comprises a rinsing phase during whichthe first valve is closed whereas the second and third valves are open,the motor being activated step by step so as to push back the rinsingliquid contained in the two syringes in the direction of the drawing-offmeans.
 5. Device according to claim 4, wherein the said drawing-offcycle comprises a return to zero phase comprising the filling of thechambers with the rinsing liquid, the first two valves being openwhereas the third valve is closed, the motor rotating in a negativedirection so as to bring back the pistons below the “zero” position,followed by a phase for evacuating air from the drawing-off means byopening the second and third valves and by closing the first valve, themotor rotating in a positive direction so as to provoke a flowing backof the rinsing liquid towards the drawing-off means and to bring backthe pistons into an idle position, the third electrovalve then beingclosed whereas the first electrovalves are open.
 6. Device according toclaim 1, wherein, during said drawing-off cycle, at the time of the saidtransitory phase, the second and third valves are open and the secondvalve is closed, and at the time of said end of sample taking phase, thevalve is closed and the motor starts an inversion of direction ofrotation transitory phase.
 7. Device according to claim 6, wherein saiddrawing-off cycle comprises a flowing back sequence with first a flowingback of the rinsing liquid into the two chambers, and secondly a flowingback of the sample into the analysis receptacle, this sequencecomprising the following successive phases: a phase where the first andsecond valves are open and the third valve is closed to allow a flowingback of the rinsing liquid contained in the chambers towards thereceptacle, a transitory play adjustment phase in which the second valveis closed, the first valve remaining open and the third valve closed, aphase in which the third valve is open whereas the first valve staysopen and the second valve is closed to enable the product to flow backinto the analysis receptacle, a phase for controlling the zero positionof the motor.
 8. Device according to claim 7, wherein the saiddrawing-off cycle comprises a rinsing sequence during which the liquidcontained in the pipette is pushed back into the rinsing well, thesecond and third valves being open whereas the first valve is closed,the motor being activated step by step so as to obtain a flowing back inseveral stages.
 9. Device according to claim 8, wherein the saiddrawing-off cycle comprises a phase for return to an initial statecomprising: the filling of the chambers with the rinsing liquid, thefirst and second valves being open and the third valve closed, the motorrotating in a negative direction as far as a position slightly below thezero level, a zero control phase during which the motor is driven inrotation in a positive direction until the zero position is detected, aplay adjustment phase in which the first valve is closed again and themotor is driven in rotation in a negative direction until it comes backto a position situated slightly below the zero level. a final phase forreturn to an initial state in which the first and third valves are openwhereas the second valve is closed, the motor being at a dead stop. 10.Device according to claim 1, wherein the cylindrical cavities of the twopumping units are embodied in a given material block.
 11. Deviceaccording to one of the claim 1, wherein the said motorisation comprisesa motor driving a pinion which gears with a rack integral with saidactivation member.
 12. Device according to claim 1, wherein the upperextremities of the cylindrical cavities and the rod/piston assembliesare conical.
 13. Device according to claim 12, wherein the conical shapeof the smallest cylindrical cavity communicates directly with the pipeconnected to the drawing-off means.
 14. Device according to claim 1,wherein the said pumping units consist of modules each comprising a bodyhaving two parallel assembling faces into which a traversing pipe opensin communication with said cylindrical cavity and having one portionable to be sealed off by an electrovalve, the orifices of said pipebeing equipped with connection means making it possible to provide asealed connection with a corresponding orifice of another module whenthe two modules are assembled to each other via their assembling facesand fixed in this position with the aid of fixing means, said orificesbeing able to be moreover connected, either to the rinsing liquid intakepipe or to the pipe connected to the drawing-off means.
 15. Deviceaccording to claim 14, wherein each of the modules comprises a pipe incommunication with the cylindrical cavity and which opens outside via anorifice constituting a parallel outlet, said pipe being able to besealed off by an electrovalve.
 16. Device according claim 1, whereinsaid electrovalves and said motorisation are controlled by a processorreceiving information relating to the position of the rod/pistonassemblies.
 17. Device according to claim 16, wherein said informationis obtained with the aid of an optical fork associated with said rack.